1. Field of the Invention
The present invention pertains to storage and retrieval of information relative to rotating storage medium, and particularly to locating an index mark on rotating storage medium.
2. Related Art and other Considerations
Typically a rotating magnetic storage medium is pre-recorded upon manufacture to include certain servo information, usually in the form of servo wedges. The servo information includes an index mark which appears in only one servo wedge per track and which is important for providing a precise position reference on the rotating storage medium. As explained below, locating the index mark can be problematic in some circumstances.
In accordance with prior art formatting for rotating magnetic storage medium, servo information is recorded in each of plural servo wedges located at equal radial intervals on the medium. Each prior art servo wedge begins with a preamble, which is followed by a synchronization mark and a servo data field. One value in the servo data field (e.g., the first bit of servo data field) is set to a value to indicate whether the servo wedge is the index wedge for the track.
It will be appreciated that if there is an error in reading the index mark (e.g., in reading the bit of servo data field used for signifying the index mark), the index mark-search operation is frustrated. For this reason, prior art rotating magnetic storage drives typically rotate the medium several times to ensure consistent results for locating the index mark. Unfortunately, the additional rotations and reading of several tracks to check for consistent indication of the index mark cost time.
What is needed, therefore, and an object of the present invention, is method and apparatus for easily locating an index mark on rotating storage medium, as well as a rotating storage medium pre-formatted to facilitate efficient location of the index mark.
A rotating storage medium has an index mark-indicating sequence recorded thereon, the index mark-indicating sequence comprised of bits distributed over plural servo wedges. The index mark-indicating sequence is comprised of at least one bit in each of a specified number of plural servo wedges per track, the specified number forming a subset of the servo wedges of the rotating storage medium. The specified number (subset) of plural servo wedges per track can be a predetermined number of consecutive servo wedges positioned immediately before an index mark, or the subset can be offset from (before or after) the servo wedge which includes the index mark by a predetermined number of servo wedges.
An index mark search operation is performed as an aspect of a method of the invention for operating a drive. In accordance with the index mark search operation, a sequence is dynamically formed from bits extracted from each of plural servo wedges read from the rotating storage medium. As a new servo wedge is read, a bit from the new servo wedge is included in the sequence, with an oldest bit of the sequence being shifted out of the sequence. Upon the reading of each servo wedge, the contents of the sequence is compared with a predefined reference index mark-indicating sequence. The comparison is used to locate an index mark on the rotating storage medium.
The invention further encompasses a drive for a rotating storage medium. The drive comprises a transducer for reading servo wedges pre-recorded on the medium. A register is employed to store a sequence of servo information exacted from predetermined positions of plural servo wedges read by the transducer. A comparitor compares contents of the register with a predefined index mark-indicating sequence for ascertaining an index mark recorded on the medium.
In another aspect, the invention concerns a controller for a drive which reading servo wedges pre-recorded on the medium. The controller includes the register in which the sequence is stored and the comparitor, as well as an input information (buffer) manager which, for each of plural servo wedges read by the transducer, makes an entry in the shift register.
In yet another aspect, the invention involves apparatus and method for pre-formatting a rotating storage medium so that the index mark-indicating sequence is distributed over plural servo wedges.
Both the synchronization marks of the plural servo wedges and the index mark-indicating sequence are preferably fault tolerant. The bits used to form the acquired sequence (which is checked during the index mark search operation against the predefined reference index mark-indicating sequence) are preferably recorded in synchronization marks of the specified number of plural servo wedges per track, and particularly in a sequence entry bit thereof. In view of the fault tolerant nature of the synchronization mark, the sequence entry bit from which the sequence entry value is acquired is chosen so that a nominal synchronization mark has a first value (e.g., zero) in the sequence entry bit, while the variant but acceptable synchronization mark has a second value (e.g., one) in the sequence entry bit. The values from the sequence entry bits of consecutive servo wedges are thus used to form a value in a register, which value is checked against the predetermined reference index mark-indicating sequence.